Related Stories

Buzz off For those mosquito magnets amongst us, help is at hand with US scientists developing a way to stop the irritating insects zeroing in on human hosts.

The US researchers have modified mosquitoes to impair their sense of smell, making it harder for them to target people.

While it may make for more relaxing barbecues the development also has implications for the management of mosquito-borne diseases such as malaria and Dengue fever.

The two main carriers of the diseases, Anopheles gambiae and Aedes aegypti, have evolved to have a strong preference for human odours.

Lead researcher Professor Leslie Vosshall, of Rockefeller University in New York, says this preference is an important factor in making both species of mosquitoes dangerous vectors of infectious disease.

"Because they will tend to go from human to human to human without feeding on other non-human animals, they are perfectly positioned to transmit disease," she says.

The study, published today in Nature, examines how mosquitoes discriminate between humans and other warm-blooded animals.

A key focus of the work is an olfactory co-receptor known as orco, which is necessary to assemble odour-specific proteins (ORs) into an odour-smelling machine.

"When orco is mutated, the mosquito can no longer build olfactory receptors and is therefore impaired in discriminating human from non-human smell," says lead author Professor Leslie Vosshall.

Disrupted genes

The team used a technology called zinc-finger nucleases, which uses DNA-binding proteins to disrupt the orco gene in the mosquito.

The biggest effect was found in the absence of carbon dioxide, says Vosshall. Without CO2 the orco mutants could smell neither honey - an important food source - nor human scent.

"When we add back CO2, the mosquitoes are strongly attracted to human scent, but fail to properly discriminate between human and non-human scent," says Vosshall.

"This tells us that CO2 is a very important factor in mosquito host-seeking behaviour."

Vosshall says the "main take-away" of the paper is that without orco, mosquitoes show reduced preference for humans.

"The next steps would be to figure out which specific odour-specific ORs are detecting human and non-human odours."

The researchers also looked at the impact of the orco gene on sensitivity to the insect repellent DEET.

"Our mutant mosquitoes are completely insensitive to the airborne effects of DEET," says Vosshall. "They fly through a cloud of DEET coming from someone's arm and only realise the DEET is there after they land and touch the skin."

This shows DEET acts on two fronts - via smell and contact.

"Mosquitoes without orco are blind to DEET. But DEET has an important contact chemo-repellent effect. So any second-generation repellent should be careful to act both on the airborne mechanism and the skin contact mechanism if it is to be effective."